S. aureus Vaccine Failures Linked to Interleukin Production, Antibody Inactivation

Staphylococcus aureus (S. aureus; SA/Sa) is a major cause of skin and soft tissue infections that can sometimes lead to sepsis and toxic shock syndrome. And while many S. aureus vaccine candidates have been effective in mice, approximately 30 human clinical trials have so far failed. “It is a pathogen in dire need of control because it causes significant morbidity and mortality not just in the United States, but worldwide,” said George Liu, MD, PhD, professor and chief of pediatric infectious diseases at the University of California (UC), San Diego, School of Medicine and Rady Children’s Hospital-San Diego.

Liu heads a team of UC San Diego researchers that has now identified a potentially key reason for these trial failures, indicating that it may be possible to modify the vaccines to work in humans. In their study in the Journal of Clinical Investigation (JCI), titled, ”Pathobiont-driven antibody sialylation through IL-10 undermines vaccination,” Liu, together with first author Chih-Ming Tsai, and colleagues reported that S. aureus induces an overabundance of interleukin-10 (IL-10) in B cells, leading to the inactivation of antibodies, and rendering them unable to kill S. aureus. In their paper they concluded, “Overall, we demonstrate a pathobiont-centric mechanism that modulates antibody glycosylation through IL-10, leading to loss of staphylococcal vaccine efficacy.”

In a related study published the same day in Nature Communications (“Pathobiont-induced suppressive immune imprints thwart T cell vaccine responses”), Liu, along with first author Irshad Ahmed Hajam, PhD, Tsai, and a team of scientists at UC San Diego School of Medicine, reported that an overabundance of IL-10 in response to S. aureus shuts down the ability of helper T cells to fight the pathogen. In the paper, the team proposed that “… IL-10 secreting, SA-experienced CD4+ T cell imprints represent a staphylococcal immune escaping mechanism that needs to be taken into consideration for future vaccine development.”

 

The majority of us are colonized with Staphylococcus aureus (SA) at an early age, the authors noted in their Nature Communications paper. “SA is a pathobiont that colonizes and infects humans from early infancy, with up to fifty percent of infants having been exposed to SA by the age of six months.” The team reporting in JCI further noted, “As a superbly adapted ‘pathobiont,’ Sa has evolved intricate mechanisms facilitating its coexistence with the host.”

S. aureus shares a long history with humans, Liu pointed out. “For a bacterium to readily live in our nose and gut, it needs to develop a strategy that effectively dampens the immune response to be able to survive.” S. aureus is a leading cause of human bacterial skin, soft tissue, and invasive infections, and poses a significant threat to public health, made worse by the spread of methicillin-resistant Staphylococcus aureus bacteria (MRSA). As a major opportunistic pathogen, S.aureus has been the target of vaccine research efforts since 1903, the team pointed out. However, “Despite the demonstrated efficacy of numerous anti-Sa vaccine candidates in naïve mice, approximate­ly 30 human clinical trials have failed to yield a functional vaccine, for unclear reasons,” the JCI authors noted. “Laboratory animals are predominantly naive to human Sa, which stands in stark contrast to humans who encounter Sa from early infancy onward.”

For the most part, the bacterium doesn’t cause harm, but a previous study led by Tsai, an assistant project scientist in Liu’s lab, showed that this early exposure fools our immune cells into producing modified antibodies that fail to mount an effective defense against S. aureus. What’s more, the bacteria retain a “memory” of those non-protective antibodies that can be brought back during later infections.

 

Tsai says that’s why vaccine candidates that have worked well in mice with no previous exposure to the pathogen have failed to protect humans from new encounters with S. aureus. When the researchers exposed mice to human S. aureus antibodies before vaccination—to replicate early human experience with the bacteria—the vaccine no longer worked. Their findings indicated that Sa infections generate these nonprotective “antibody imprints” that, upon anti-S. aureus vaccination, lead to suboptimal vaccine responses. “Thus, imprinting emerges as a plausible explanation for the widespread failure of Sa vaccines,” the authors of the JCI study stated.

Tsai, Liu, and colleagues sought to understand what renders the S. aureus antibodies useless at fighting the pathogen after vaccination. “While we had established that humoral imprinting interferes with effective vaccination, the mechanism by which the pathobiont Sa renders anti-Sa antibodies ineffectual across a spectrum of vaccines remained unclear,” they stated in JCI.

For their newly described study, the researchers exposed mice to S. aureus, and later inoculated them with iron surface determinant B (IsdB) vaccine, which had previously been shown to confer immunity to S. aureus in mice that were naive to the bacteria.

The team found that B cells—white blood cells that make antibodies—secrete an abundance of IL-10 when challenged with S. aureus for a second time. Within the B cells, IL-10 directs the enzymes to add a sugar called sialic acid to the antibodies’ Fc region, which is responsible for generating an appropriate immune response. With the sugar abundantly present, the anti-staphylococcal activity of antibodies produced by the B cells was neutralized, making them incapable of killing the pathogen. “… we report a mechanism by which the pathobi­ont Sa induces anti-inflammatory cytokine IL-10 to modify sialyla­tion on anti-Sa antibodies, thereby neutralizing their host-pro­tective activity,” the researchers stated in JCI. “This nonprotective humoral response is recalled during vaccination, resulting in an ineffective vaccine response.”

Tsai explained, “The IL-10 is helping make tons of this sugar type and by doing so, it’s turning off our immune system.” However, the researchers also found that blocking IL-10 at the time of immunization restored vaccine efficacy. “The same vaccine that didn’t work before now works perfectly in mice,” Tsai added.

 

While the JCI study focused on the role of IL-10 in B cells, for the study reported in Nature Communications, Hajam, an assistant project scientist in Liu’s lab, and colleagues examined how S. aureus interacts with CD4+ helper T lymphocytes, white blood cells that detect infections and activate other immune cells to attack and kill pathogens.

The researchers found that like B cells, helper T cells also secrete an overabundance of IL-10 in response to S. aureus in mice previously exposed to and later vaccinated for S. aureus.

IL-10 shuts down the ability of the helper T cells to produce interleukin-17 (IL-17A), a cytokine that is particularly effective at fighting S. aureus infections. “In this study, we show that Alum/IsdB vaccine recalls CD4+ T cells that secrete abundant IL-10,” the team wrote in the Nature Communications report. “IL-10 in turn suppresses host-protective IL-17 responses, consistent with our prior finding of SA evasion of IL-17 responses through IL-10.” But by blocking IL-10 or adding a substance called CAF01—known to enhance vaccine efficacy by increasing the response of T cells to microbial infections—the researchers were able to restore IL-17A levels. “Staphylococcal vaccine interference can be overcome by applying potent IL-17-inducing adjuvants with vaccine antigens in SA-experienced hosts,” the team commented. “Adding CAF01 during vaccination helped turn the ineffective IsdB vaccine into one that worked in S. aureus-exposed mice,” said Hajam. “Surprisingly, it also worked with several other failed vaccines against S. aureus.”

The findings from both studies could represent good news for human S. aureus vaccine development. Liu said it may be possible to make already-developed but failed S. aureus vaccines effective by blocking IL-10 or boosting IL-17A during vaccination. He commented that IL-10 production by a number of other microorganisms, including (Clostridioides difficile) could be a reason why promising vaccines for these conditions have failed in human clinical trials, suggesting that blocking the cytokine could restore their efficacy as well. “Among hard-to-generate vaccines, C. difficile and pathogens like M. tuberculosis, herpes simplex virus, human immunodeficiency virus, and plasmodium species are also associated with abundant IL-10 per reports,” the authors wrote in Nature Communications.

 

“The role of IL-10 in circumventing vaccine protection against these pathogens would be worth exploring.” In their JCI paper, the scientists further pointed out that antibodies from cystic fibrosis subjects are also nonfunctional, and the peripheral blood mononuclear cells from cystic fibrosis patients demonstrate induction of high IL-10 levels, when compared with cells from normal individuals. “These observations reinforce the link between sialylation and IL-10 in pathobionts other than Sa,” they wrote. “These discoveries should prompt a more comprehensive assessment of human anti-Sa antibodies, extending the investigations to include antibodies against other pathobionts and antibodies from unsuccessful human vaccine trials.”